Nonvolatile semiconductor memory device and method of manufacturing the same

ABSTRACT

A nonvolatile semiconductor memory device comprises: element isolation insulating films formed in a semiconductor substrate in a first direction; and element regions formed in a region sandwiched by the element isolation insulating film, with MONOS type memory cells. The MONOS type memory cell comprises: a tunnel insulating film disposed on the element region; a charge storage film disposed continuously on the element regions and the element isolation insulating films. The charge storage film comprises: a charge film disposed on the element region and having a certain charge trapping characteristic; and a degraded charge film disposed on the element isolation insulating film and having a charge trapping characteristic inferior to that of the charge film. The degraded charge film has a length of an upper surface thereof set shorter than a length of a lower surface thereof in a cross-section along the first direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2009-155825, filed on Jun. 30,2009, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transistor structure utilized in anonvolatile semiconductor memory device.

2. Description of the Related Art

Large capacity storage devices such as NAND flash memory are widely usedin consumer-oriented household electrical goods such as storage cards.Since the storage devices used many of these household electrical goodsrequire high reliability and long-term storage stability.

There are typically two kinds of cell structure in NAND flash memory,namely, floating gate (hereafter referred to as “FG”) type structure andMONOS (Metal-Oxide-Nitride-Oxide-Semiconductor) type structure. The FGtype structure and MONOS type structure are described below.

In the FG type structure, a charge is stored in a floating gate composedof a conductor (for example, polysilicon) disposed on a gate insulatingfilm (tunnel insulating film) of a MOSFET. The FG type structure storesinformation based on the difference in the amount of charge stored.

On the other hand, in the MONOS type structure, a charge is stored in atrap level in a charge storage film (for example, a silicon nitride(SiN) film) instead of in the floating gate. The MONOS type structure issimilar to the FG type structure in that information is stored based onthe difference in the amount of charge stored.

The FG type structure is adopted as the cell structure in NAND flashmemory, because the FG type structure has good charge storing (hereafterreferred to as “retention”) characteristic. On the other hand, the MONOStype structure allows a greater degree of thinning in its charge storagefilm. As a result, shifting of a threshold voltage in memory celltransistors caused by capacitance coupling with adjacent cells can bereduced. Amemory cell transistor with a MONOS type structure is reportedin “Charge Trapping Memory Cell of TANOS (Si-Oxide-SiN—Al2O3—TaN)Structure Compatible to Conventional NAND Flash Memory” (IEEE NVSMW2006. 21st Volume, Issue 2006 pp. 54-55), for example. This memory celltransistor utilizes a tantalum nitride film (TaN) for the control gateelectrode, an alumina film (Al₂O₃) that is a high dielectric constantinsulating film for the block insulating film, a silicon nitride filmfor the charge storage film, and a silicon oxide film (SiO₂) for thetunnel insulating film).

The retention characteristic of the memory cell transistor with theMONOS type structure is required to improve.

The cause of deterioration in the retention characteristic is consideredto lie in the method of processing of the tunnel insulating film and thecharge storage film. In fact, the retention characteristic is reportednot to deteriorate when the tunnel insulating film and the chargestorage film are not processed (refer to “Study of Local Trapping andSTI Edge Effects on Charge-Trapping NAND Flash”, Electron DevicesMeeting, 2007. IEDM 2007. IEEE International, pp. 161-164). In aconventional method of manufacturing a memory cell transistor having aMONOS type structure, Reactive Ion Etching (hereafter referred to as“RIE”) is used to form element isolation trenches and process the tunnelinsulating film and the charge storage film at the same time. Damage tothe tunnel insulating film and the charge storage film is thought tooccur at that time. Accordingly, a method of manufacturing is reportedin which the tunnel insulating film and the charge storage film aredeposited subsequent to formation of the element isolation trench (referto “Self Aligned Trap-Shallow Trench Isolation Scheme for theReliability of TANOS (TaN/AlO/SiN/Oxide/Si) NAND Flash Memory”, IEEENVSMW 2007. 22st Volume, Issue 2007 pp. 110-111). However, whenmanufacture is performed by this method, the problem arises that thecharge storage films of adjacent memory cell transistors become joinedin structure, whereby charge in a charge storage film shifts to anadjacent memory cell transistor.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, anonvolatile semiconductor memory device comprises: a semiconductorsubstrate; element isolation insulating films being along a firstdirection in the semiconductor substrate with a certain spacing therebetween and reached to a certain depth from a surface of thesemiconductor substrate; element regions sandwiched by the elementisolation insulating film, with MONOS type memory cells disposed on anupper surface thereof, each of the MONOS type memory cells comprising: atunnel insulating film formed on one of the element regions; a chargestorage film disposed continuously on the element regions and theelement isolation insulating films; a block insulating film disposed onthe charge storage film; and a control gate electrode disposed on theblock insulating film, the charge storage film comprising: a charge filmdisposed on the one of the element regions and having a certain chargetrapping characteristic; and a degraded charge film disposed on one ofthe element isolation insulating film and having a charge trappingcharacteristic inferior to that of the charge film, and the degradedcharge film having a length of an upper surface thereof set shorter thana length of a lower surface thereof in a cross-section along the firstdirection.

In accordance with a second aspect of the present invention, anonvolatile semiconductor memory device comprises: a semiconductorsubstrate; element isolation insulating films being along a firstdirection in the semiconductor substrate with a certain spacing therebetween and reached to a certain depth from a surface of thesemiconductor substrate; and element regions sandwiched by the elementisolation insulating film, with floating gate type memory cells disposedon an upper surface thereof, each of the floating gate type memory cellscomprising: a tunnel insulating film disposed on one of the elementregions; a floating gate disposed on the tunnel insulating film, havingan upper surface higher than an upper surface of the element isolationinsulating films; an inter-gate multilayer film disposed continuously onthe element isolation insulating films and the floating gate; and acontrol gate electrode disposed on the inter-gate multilayer film, theinter-gate multilayer film comprising a first inter-gate film and asecond inter-gate film formed on the first inter-gate film, and thesecond inter-gate film comprising: a side surface portion disposed on aside surface of the floating gate and configured from a material havinga dielectric constant higher than that of the first inter-gate film; andan upper surface portion disposed on the element isolation insulatingfilms and having fewer charge traps than the side surface portion.

In accordance with a third aspect of the present invention, a method ofmanufacturing a nonvolatile semiconductor memory device comprises:forming a plurality of element isolation trenches along a firstdirection with a certain spacing therebetween, the plurality of elementisolation trenches reaching a certain depth from a surface of asemiconductor substrate; filling the element isolation trench with aninsulating film including an oxidizing agent, thereby forming an elementisolation insulating film; forming a tunnel insulating film of a MONOStype memory cell on a surface of an element region, the element regiondivided by the element isolation insulating film; forming a chargestorage film of the MONOS type memory cell continuously on the elementregion and the element isolation insulating film; and using heattreatment to diffuse the oxidizing agent included in the elementisolation insulating film and oxidize the charge storage film formed onthe element isolation insulating film, thereby forming a degraded chargefilm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an equivalent circuit diagram of a cell array in a nonvolatilememory in accordance with a first embodiment of the present invention.

FIG. 2 is a plan view of the cell array in the nonvolatile memory ofFIG. 1.

FIGS. 3A and 3B are cross-sectional views taken along the line I-I′ ofFIG. 2.

FIGS. 4A-4D are cross-sectional views showing manufacturing processes ofa memory cell transistor in the nonvolatile memory of FIG. 1.

FIG. 5 is a graph of moisture (H₂O) deposition amount subsequent to heattreatment according to material of an element isolation insulating film.

FIG. 6 is a cross-sectional view describing formation of a channelregion in a peripheral transistor of the nonvolatile memory of FIG. 1.

FIG. 7 is another cross-sectional view taken along the line I-I′ of FIG.2.

FIG. 8 is an equivalent circuit diagram of a memory string in anonvolatile memory in accordance with a second embodiment of the presentinvention.

FIGS. 9A-9C are cross-sectional views of the memory string in thenonvolatile memory of FIG. 8.

FIG. 10 is a cross-sectional view of a cell array in a nonvolatilememory in accordance with a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of a nonvolatile semiconductor memory device and a method ofmanufacturing the same in accordance with the present invention aredescribed in detail below with reference to the drawings.

First Embodiment

The nonvolatile memory in accordance with a first embodiment of thepresent invention comprises a NAND flash memory, and an equivalentcircuit diagram of a cell array in this nonvolatile memory is as shownin FIG. 1.

That is, the cell array in the nonvolatile memory of the presentembodiment comprises a plurality of word lines WL1-WLn (where n isinteger greater than or equal to 1) and a plurality of bit lines BL1-BLm(where m is an integer greater than or equal to 1) that intersect eachother. In addition, this cell array includes a source line SL, a sourceside select gate line SGS, and a drain side select gate line SGDextending in a word line WL direction. Furthermore, this cell array hasarranged therein a plurality of memory strings MS1-MSm, each of whichcomprises a plurality of memory cell transistors MT1-MTn connected inseries. Here, the memory cell transistor MTi (where i is an integer from1 to n) is configured by a transistor with a MONOS type structure to bedescribed hereafter, and the m memory cell transistors MTi belonging tothe memory strings MSj (where j is an integer from 1 to m) have theircontrol gate electrodes commonly connected to the word line WLi. Inaddition, a select transistor ST1 is provided to one end of each of thememory strings MSj, the memory string

MSj being connected to the source line SL via this select transistorST1. Meanwhile, a select transistor ST2 is provided to the other end ofeach of the memory strings MSj, the memory string MSj being connected tothe bit line BLj via this select transistor ST2. Here, the m selecttransistors ST1 and ST2 have their gate electrodes commonly connected tothe source side select gate line SGS and the drain side select gate lineSGD, respectively.

[Structure of Memory Cell Transistor]

FIG. 2 is a plan view of this cell array, FIG. 3A is a cross-sectionalview taken along an I-I′ direction of FIG. 2, and FIG. 3B is an enlargedview of a region surrounded by a dotted line of FIG. 3A. Note that across-section taken along an II-II′ direction of FIG. 2 is identical tothat of an ordinary NAND flash memory, and is therefore not shown.

The memory cell transistor MTi of the present embodiment is configuredby a transistor with a MONOS type structure as mentioned above.

Specifically, the memory cell transistor MTi is disposed on a silicon(Si) substrate 101 that is a semiconductor substrate. A plurality ofelement isolation trenches 102 are disposed in this silicon substrate101 so as to extend in a bit line BL direction that is a first directionand so as to reach a certain depth from a surface of the siliconsubstrate 101. Furthermore, an element isolation insulating film 103formed, for example, using ALD (Atomic Layer Deposition) and constitutedfrom SiO₂ (ALD-SiO₂), that is, a so-called STI (Shallow TrenchIsolation) is formed in these element isolation trenches 102. Note that,in addition to ALD-SiO₂, the following may also be utilized as amaterial of the element isolation insulating film 103, namely NSG (NonDoped Silicate Glass), BPSG (Boron Phosphorous Silicon Glass), HTO (HighTemperature Oxide), TEOS (Tetra Ethyl Ortho Silicate), PSZ(Polysilazane), and so on. Now, in the following description, a regionof the silicon substrate 101 between adjacent element isolationinsulating films 103 is called an element region AA. Disposed on anupper surface of this element region AA is a tunnel insulating film 104of a certain thickness (for example, 2-10 nm). This tunnel insulatingfilm 104 may have a single-film structure configured by a silicon oxidefilm (SiO₂). Alternatively, it may have a stacking structure of thelikes of a silicon nitride film (SiN)/silicon oxide film, silicon oxidefilm/silicon nitride film/silicon oxide film, silicon oxide film/highdielectric constant film/silicon oxide film, or high dielectric constantfilm/silicon oxide film.

In addition, a charge storage film 105 including a charge trap isstacked in a continuous manner on the element region AA and the elementisolation insulating film 103. This charge storage film 105 comprises acharge film 105 a having a certain charge trapping characteristic, and adegraded charge film 105 b having a charge trapping characteristic andcharge mobility inferior to that of the charge film 105 a. As shown inFIG. 3A, this degraded charge film 105 b reaches from a lower surface toan upper surface of the charge storage film 105 and separates adjacentcharge films 105 a. Moreover, this degraded charge film 105 b is formedby an oxidizing agent (for example, moisture (H₂O)) included in theelement isolation insulating film 103 being diffused by heat andoxidizing a material of the charge storage film 105. Since thisoxidizing agent is diffused isotropically from the element isolationinsulating film 103, a cross-section in the I-I′ direction of thedegraded charge film 105 b becomes near-trapezoidal in shape wherein alength H101 of an upper surface of the degraded charge film 105 b isshorter than a length H102 of a lower surface of the degraded chargefilm 105 b in contact with the element isolation insulating film 103, asshown in FIG. 3A. In the detail, a boundary between the charge film 105a and the degraded film 105 b in a cross section along the I-I′direction has an upwardly-convex arc-like shape, as shown in FIG. 3B.The arc-like boundary is formed with its center near an upper end P of aboundary between the element isolation insulating film 103 and thetunnel insulating film 104 in the I-I′ cross section, and has a convexshape toward the charge film 105 a. Here, the charge film 105 a may havea single-film structure configured by a silicon nitride film or ahafnium aluminate film (HfAlO). Alternatively, it may have a multilayerfilm structure including a high dielectric constant film with adielectric constant higher than that of a silicon oxide film. The highdielectric film includes an alumina film (Al₂O₃), magnesium oxide film(MgO), strontium oxide film (SrO), barium oxide film (BaO), titaniumoxide film (TiO₂), tantalum oxide film (Ta₂O₅), barium titanium oxidefilm (BaTiO₃), barium zirconium oxide film (BaZrO), zirconium oxide film(ZrO₂), hafnium oxide film (HfO₂), yttrium oxide film (Y₂O₃) zirconiumsilicate film (ZrSiO), hafnium silicate film (HfSiO), lanthanumaluminate film (LaAlO) and the like. Specifically, the charge film 105 amay employ a multilayered film structure formed of a silicon nitridefilm, a high dielectric constant film, and a silicon nitride film whichare stacked on the tunnel insulating layer 104 in this order.Alternatively, it may employ a multilayered film structure of hafniumaluminate film/high dielectric constant film/silicon nitride film,silicon nitride film/high dielectric constant film/hafnium aluminatefilm, or hafnium aluminate film/high dielectric constant film/hafniumaluminate film. In addition, regarding the silicon nitride film, eithera ratio of x or y in Si_(x)N_(y) may be increased compared to the other.

Furthermore, a block insulating film 106, a control gate electrode 107,and a spacer 108 are stacked sequentially on the charge storage film105. Here, in addition to a silicon oxide film, a film having adielectric constant higher than a silicon oxide film may also beutilized as the block insulating film 106, The high dielectric constantfilm may include an alumina film, magnesium oxide film, strontium oxidefilm, silicon nitride film, barium oxide film, titanium oxide film,tantalum oxide film, barium titanium oxide film, barium zirconium oxidefilm, zirconium oxide film, hafnium oxide film, yttrium oxide film,zirconium silicate film, hafnium silicate film, or lanthanum aluminatefilm and the like. In addition, the block insulating film 106 may have amultilayer film structure including these high dielectric constantfilms. Specifically, the block insulating film 106 may employ amultilayered film structure formed of a silicon oxide film, a highdielectric constant film, and a silicon oxide film which are stacked onthe charge storage film 105 in this order. Alternatively, it may employa multilayered film structure of silicon oxide film/high dielectricconstant film, high dielectric constant film/silicon oxide film, or highdielectric constant film/silicon oxide film/high dielectric constantfilm. Moreover, the control gate electrode 107 may have a single-filmstructure configured by polysilicon, a metal compound of silicon(silicide), a metal oxide, or a metal (tungsten (W), tantalum siliconnitride (TaSiN), tantalum (Ta), titanium silicide (TiSi), titaniumnitride (TiN), cobalt (Co), platinum (Pt) or the like). Alternatively,it may have a multilayer film structure of these.

[Method of Manufacturing Memory Cell Transistor MTi]

Next, a method of manufacturing the memory cell transistor MTi of thepresent embodiment is described with reference to FIGS. 4A-4D. The caseis described here where an ALD-SiO₂ film is used for the elementisolation insulating film 103, a silicon oxide film is used for thetunnel insulating film 104, and a silicon nitride film is used for thecharge storage film 105.

As shown in FIG. 4A, RIE is used on a silicon substrate 151 (siliconsubstrate 101 shown in FIGS. 3A and 3B) to form a plurality of elementisolation trenches 152 (element isolation trench 102 of FIGS. 3A and 3B)with a certain spacing the rebetween (for example, 50 nm) in the bitline direction and reaching a certain depth (for example, 200-400 nm)from a surface of the semiconductor substrate 151. The element isolationtrench 152 is filled with an ALD-SiO₂ film 153 including moisture (H₂O)that is an oxidizing agent, thereby forming the element isolationinsulating film 103 shown in FIG. 3. Here, CMP may be used to planarizean upper surface of the ALD-SiO₂ film 153 if required. In addition, asurface of the ALD-SiO₂ film 153 may be etched back to set a height of asurface of the element isolation insulating film 103 lower than a heightof a surface of the element region AA. In this case, an opposing area ofthe charge film 105 b and the control gate electrode 107 becomes large,and capacitance coupling between the charge film 105 b and the controlgate electrode 107 can therefore be increased.

As shown in FIG. 4B, an upper surface of the element region AA of thesilicon substrate 151 is, for example, oxidized to form a silicon oxidefilm 154 that becomes the tunnel insulating film 104.

As shown in FIG. 4C, a silicon nitride film 155 that becomes the chargestorage film 105 is deposited on the ALD-SiO₂ film 153 and the siliconoxide film 154.

As shown in FIG. 4D, heat treatment is applied to the ALD-SiO₂ film 153,thereby diffusing the moisture (H₂O) included in the ALD-SiO₂ film 154into the silicon nitride film 155. This result in a film quality of aportion of the silicon nitride film 155 on the upper surface of theALD-SiO₂ film 153 being oxidized. That is, the silicon nitride film 155is oxidized to become a silicon oxynitride film. In this case, amaterial of the degraded charge film 105 b is the silicon oxynitridefilm. A degraded silicon nitride film 155 b formed by this oxidation isthe degraded charge film 105 b shown in FIG. 3. On the other hand, asilicon nitride film 155 a which is an unoxidized portion of the siliconnitride film 155 is the charge film 105 a shown in FIG. 3.

Films that become the block insulating film 106, the control gateelectrode 107, and the spacer 108 are formed sequentially on the siliconnitride film 155 a and the degraded silicon nitride film 155 b. Thememory cell transistor MTi of the MONOS type structure shown in FIG. 3is thereby completed.

Note that, although, in the above-described method of manufacturing,formation of the degraded silicon nitride film 155 b is performed priorto formation of the block insulating film 106, this degraded siliconnitride film 155 b may be formed subsequent to formation of the blockinsulating film 106 and control gate electrode 107 either before orafter formation of the spacer 108.

A material of the element isolation insulating film 103 is nowdescribed. FIG. 5 is a table comparing moisture (H₂O) deposition amountsubsequent to heat treatment for each of various materials. FIG. 5 hereis a table showing a relative comparison of moisture (H₂O) depositionamount subsequent to heat treatment where HTO is assumed to be 1. Notethat “Low Temp.” indicates the moisture deposition amount when heattreatment is performed between room temperature and about 450 degrees,and “High Temp.” indicates the moisture deposition amount when heattreatment is performed at 450 degrees and above. It is clear from FIG. 5that moisture (H₂O) deposition amount subsequent to heat treatmentincreases in order of HTO, TEOS and ALD-SiO₂, NSG, and PSZ.Alternatively, it may have a multilayer film structure of these. PSZ orNSG could therefore be said to be preferable from among these materials,if the heat process subsequent to formation of the degraded siliconnitride film 155 b is considered.

[Method of Manufacturing Peripheral Transistor]

This concludes description of the method of manufacturing the memorycell transistor, but it should be noted that the heat treatment appliedduring formation of the degraded silicon nitride film 155 b in theabove-described method of manufacturing imparts effects on ionconcentration or the like in a channel regions of peripheral transistorssuch as select transistors ST1, ST2, and the like.

Accordingly, a method of forming a channel region of a peripheraltransistor that takes into account the effects of heat treatment isdescribed below.

There are two methods of forming a channel region of a peripheraltransistor, namely, a method in which the channel region is formed priorto formation of the element isolation insulating film 103 and a methodin which the channel region is formed subsequent to formation of theelement isolation insulating film 103.

In the former method, first, ion implantation is performed on thesilicon substrate 171; then, the ALD-S10 ₂ film 153, silicon oxide film154, and silicon nitride film 155 of the memory cell transistor MTi areformed, and heat treatment is applied. As a result, the dosing amount ofions implanted into the silicon substrate 171 is made larger than in thecase where no heat treatment is applied. Moreover, since this heattreatment causes a profile to extend, a range of ion implantation is setnarrower in advance than in the case where no heat treatment is applied.

In the latter method, as shown in FIG. 6, first, heat treatment is usedto form the degraded silicon nitride film 155 b; then, upon applicationof resist 180 on the element isolation insulating film 173, ions areimplanted from above the gate electrode 177. In such a case where ionsare implanted subsequent to the heat treatment, effects due to heattreatment have no influence on ion implantation conditions. Note that athickness of the gate insulating film 174 of the peripheral transistoris 20-50 nm in the case of a high voltage transistor (HVTr) and 5-10 nmin the case of a low voltage transistor (LVTr).

As is clear from the above, since no element isolation insulating film103 is formed using RIE prior to formation of the tunnel insulating film104 and charge film 105 a in the present embodiment, the presentembodiment may avoid RIE-induced damage to side surfaces of the tunnelinsulating film 104 and charge film 105 a. In addition, the degradedcharge film 105 b having an inferior charge-trapping characteristic andan inferior charge mobility is interposed between the charge films 105 aof adjacent memory cell transistors MTi, thereby preventing chargeshifting between the charge films 105 a. As a result, the retentioncharacteristic can be improved in comparison with a structure where thecharge film is separated by RIE. Moreover, charge shifting betweencharge films 155, which is a concern in a conventional memory celltransistor having a structure where charge films are joined, does notreadily occur.

In addition, since the degraded charge film 105 b is formed by diffusingthe oxidizing agent included in the element isolation insulating film103, the memory cell transistors MTi aligned in the I-I′ direction canbe isolated in a self-aligning manner. That is, misalignment of theelement region AA and the charge film 105 a does not readily occur.

Note that, although the memory cell transistor MTi shown in FIG. 3 has astructure in which the degraded charge film 105 b extends from a lowersurface of to reach an upper surface of the charge storage film 105 andcompletely isolates between adjacent charge films 105 a, a degradedcharge film 105 b′ may alternatively be formed with a certain thicknessextending from a lower surface of but not reaching an upper surface of acharge storage film 105′, as shown in FIG. 7. In this case, a boundarybetween a charge film 105 a′ and the degraded charge film 105 b′ in across-section in the I-I′ direction attains substantially anupwardly-convex arc shape having an upper surface of the elementisolation insulating film 102 as a chord. If a central vicinity of thisarc is approximated to a straight line, the degraded charge film 105 b′could be said to have a shape that is near-trapezoidal wherein a lengthH101′ of this straight line is shorter than a length H102′ of the lowersurface of the degraded charge film 105 b′. Even in this case, chargeshifting between adjacent memory cell transistors MTi can be suppressedsince the charge film 105 a between adjacent memory cell transistors MTiis thin. Moreover, since a length in the I-I′ direction of the chargefilm 105 a′ becomes greater, a range in which charge can be trapped inthe charge film 105 a can be increased in comparison with the case ofFIG. 3.

Second Embodiment

A nonvolatile memory in accordance with a second embodiment of thepresent invention is configured with a structure having a cell arraystacked three-dimensionally to increase a degree of memory integration.FIG. 8 is an equivalent circuit diagram of a part of this cell array.

Specifically, this nonvolatile memory comprises, a source line SL, asource side select gate line SGS, a plurality of word lines WL1-WL4, anda drain side select line SGD. These lines are stacked from a lower layerto an upper layer, and extend in the row direction. Furthermore, formedin a layer even higher than that of this drain side select gate line SGDis a bit line BL extending in the column direction. In addition,disposed between the source line SL and the bit line BL from the lowerlayer to the upper layer are a select transistor ST1, a memory string MScomprising a plurality of memory cell transistors MT1-MT4 connected inseries, and a select transistor ST2. The source side select gate lineSGS, the word lines WL1-WL4, and the drain side select gate line SGD areconnected to gates of the select transistor ST1, the memory celltransistors MT1-MT4, and the select transistor ST2 are, respectively.

FIG. 9A is a cross-sectional view in the row direction and stackingdirection of the memory string MS portion surrounded by a dotted line ofFIG. 8; FIG. 9B is a cross-sectional view taken along the I-I′ directionof FIG. 9A; and FIG. 9C is a cross-sectional view taken along the II-II′direction of FIG. 9A.

This memory string MS has word line conductive films 207 a-207 d, aswiring conductive films that become the word lines WL1-WL4. These films207 a-207 d are disposed from the lower layer to the upper layer.Inter-word line insulating films 208 as inter-line insulating filmformed of ALD-SiO₂ are formed between the word line conductive films207, respectively. NSG, BPSG, HTO, TEOS, PSZ and the like may also beutilized here as a material of the inter-word line insulating films 208,as well as ALD-SiO₂. Alternatively, it may have a multilayer filmstructure of these. In addition, a memory hole 202 is formed topenetrate these word line conductive films 207 and inter-word lineinsulating films 208 in the stacking direction. Formed within the memoryhole 202 is a memory columnar semiconductor film 201 having acylindrical column shape. Moreover, a block insulating film 206, chargestorage film 205, and tunnel insulating film 204 are formed sequentiallyon a side wall exposed the word line conductive films 207 and theinter-word line insulating films 208 by the memory hole 202.

Now, the charge storage film 205 comprises a charge film 205 apositioned at the same height in the stacking direction as the word lineconductive films 207 and having a certain charge trappingcharacteristic, and a degraded charge film 205 b positioned at the sameheight in the stacking direction as the inter-word line insulating films208 and having a charge trapping characteristic and charge mobilityinferior to that of the charge film 205 a.

This degraded charge film 205 b is formed by an oxidizing agent (forexample, moisture (H₂O)) included in the inter-word line insulating film208 being diffused by heat and, subsequent to passing through the blockinsulating film 206, oxidizing a material of the charge storage film205. Since this oxidizing agent is diffused isotropically from theinter-word line insulating film 208, a cross-section in the rowdirection and stacking direction of the degraded charge film 205 bbecomes near-trapezoidal in shape. That is, a length H201 of a boundarybetween the degraded charge film 205 b and the tunnel insulating film204 is shorter than a length H202 of a boundary between the degradedcharge film 205 b and the block insulating film 206.

Normally, in the case of nonvolatile memory similar to that of thepresent embodiment in which the cell array is stackedthree-dimensionally, the charge films of adjacent memory celltransistors configuring the memory string MS are formed in a continuousmanner. Consequently, charge shifting occurs between adjacent memorycell transistors.

In the present embodiment, similarly in this respect to the firstembodiment, interposing the degraded charge film 205 b between adjacentcharge films 205 a prevents charge shifting between adjacent memory celltransistors MTi. As a result, the retention characteristic of the memorycell transistors MTi can be improved.

Third Embodiment

A nonvolatile memory in accordance with a third embodiment of thepresent invention is a flash memory configured from so-called floatinggate type memory cell transistors. An equivalent circuit diagram of acell array in this nonvolatile memory is similar to that of FIG. 1 andis thus omitted. FIG. 10 is a cross-sectional view taken along the I-I′line of FIG. 2.

The nonvolatile memory in accordance with the present embodiment isformed on a silicon substrate 301. A plurality of element isolationtrenches 302 are formed in this silicon substrate 301 so as to extend ina bit line BL direction that is a second direction and so as to reach acertain depth from a surface of the silicon substrate 301. An elementisolation insulating film 303, that is, a so-called STI is formed inthese element isolation trenches 302. NSG, BPSG, HTO, TEOS, PSZ and thelike may also be utilized as a material of the element insulating film303, as well as ALD-SiO₂, similarly to the element isolation insulatingfilm 103 of the first embodiment. Alternatively, it may have amultilayer film structure of these. Element regions AA are sandwichedbetween two of the isolation insulating films 303 adjacent on thesilicon substrate 301. A tunnel insulating film 304 composed of asilicon oxide film or the like sequentially on the element region AA.Disposed sequentially on an upper surface of an element region AAsandwiched between two element isolation insulating films 303 adjacentin a word line direction on the silicon substrate 301 are a tunnelinsulating film 304 configured from a silicon oxide film or the like ofa certain thickness and a floating gate 305. Disposed covering thistunnel insulating film 304 and floating gate 305 are an IPD film(inter-gate multilayer film) of a certain thickness, and a control gate309 further covering the IPD film.

Here, an upper surface of the element isolation insulating film 303 ishigher than an upper surface of the tunnel insulating film 304 and lowerthan an upper surface of the floating gate 305. In addition, the IPDfilm comprises a multilayer structure having first through thirdinter-gate films 306-308 stacked sequentially from a lower layer to anupper layer. Of these, the first and third inter-gate layers 306 and 308are formed from an insulating material of the likes of silicon oxide,for example.

On the other hand, the second inter-gate film 307 has a portion thereoffacing an upper surface and side surfaces of the floating gate 305 (sidesurface portion 307 a) formed from a high dielectric constant materialof the likes of silicon nitride, and a portion thereof facing an uppersurface of the element isolation insulating film 303 (upper surfaceportion 307 b) formed from a material oxidized the side surface portion307 a. Note that the upper surface portion 307 b can be formed byoxidizing the high dielectric constant material as a material of thesecond inter-gate film 307 stacked in advance. The oxidation isperformed using an oxidizing agent included in the element isolationinsulating film 303, similarly to the degraded charge film 105 b in thefirst embodiment.

The upper surface of the floating gate 305 at a position is higher thanthat of the upper surface of the element isolation insulating film 303in this way, it leads to the control gate 309 facing not only the uppersurface but also the side surfaces of the floating gate 305 and therebyallows the control gate 309 to secure a high coupling ratio with thefloating gate 305.

In addition, giving the above-described three-layer structure totunneling of electrons through the IPD film to be suppressed, therebypreventing discharge of electrons from the floating gate 305 to thecontrol gate 309 during a program operation and injection of electronsfrom the control gate 309 to the floating gate 305 during an eraseoperation. As a result, a leak current in the IPD film can be reducedand the retention characteristic improved over the case of an IPD filmwith a single-layer structure configured by a silicon oxide film.

Furthermore, if a material such as a silicon nitride film havingnumerous charge traps is used for the second inter-gate film 307, chargestored in one floating gate 305 shifts to other floating gates 305adjacent to the one floating gate 305 via charge traps in the uppersurface portion 307 b. As a result, an amount of charge stored in thefloating gate 305 varies, whereby data variation is caused.

Now, the upper surface portion 307 b in a portion of the secondinter-gate film 307 is disposed on the upper surface of the elementisolation insulating film 303, that is, between element regions AAadjacent in the word line WL direction. Then it is possible to preventcharge stored in one floating gate 305 shifting to other floating gates305 adjacent to the one floating gate 305 via charge traps in the uppersurface portion 307 b. As a result, the leak current in the IPD film canbe further reduced in comparison with the case where the secondinter-gate film 307 is formed entirely from the high dielectric constantmaterial.

Moreover, the side surface portion 307 a is not oxidized by theoxidizing agent included in the element isolation insulating film 303.The dielectric constant of the IPD film formed on the side surface ofthe floating gate 305 is therefore not lowered. As a result, the controlgate 309 can secure a high coupling ratio with the floating gate 305.

Other Embodiments

This concludes description of embodiments of the present invention, butit should be noted that the present invention is not limited to theabove-described embodiments, and that various alterations, additions,and so on, are possible within a range not departing from the scope andspirit of the invention.

1. A nonvolatile semiconductor memory device, comprising: asemiconductor substrate; element isolation insulating films being alonga first direction in the semiconductor substrate with a certain spacingtherebetween and reached to a certain depth from a surface of thesemiconductor substrate; and element regions sandwiched by the elementisolation insulating film, with MONOS type memory cells disposed on anupper surface thereof, each of the MONOS type memory cells comprising: atunnel insulating film disposed on one of the element regions; a chargestorage film disposed continuously on the element regions and theelement isolation insulating films; a block insulating film disposed onthe charge storage film; and a control gate electrode disposed on theblock insulating film, the charge storage film comprising: a charge filmdisposed on the one of the element regions and having a certain chargetrapping characteristic; and a degraded charge film disposed on one ofthe element isolations insulating film and having a charge trappingcharacteristic inferior to that of the charge film, and the degradedcharge film having a length of an upper surface thereof set shorter thana length of a lower surface thereof in a cross-section along the firstdirection.
 2. The nonvolatile semiconductor memory device according toclaim 1, wherein the degraded charge film separates the charge filmadjacent along the first direction.
 3. The nonvolatile semiconductormemory device according to claim 1, wherein a boundary between thecharge film and the degraded charge film in a cross-section along thefirst direction has an upwardly-convex arc-like shape.
 4. Thenonvolatile semiconductor memory device according to claim 1, whereinthe degraded charge film is formed from a material oxidized the chargefilm.
 5. The nonvolatile semiconductor memory device according to claim1, wherein the charge film is a silicon nitride film and the degradedcharge film is a material oxidized the silicon nitride film.
 6. Thenonvolatile semiconductor memory device according to claim 1, whereinthe charge film includes a high dielectric constant film having adielectric constant higher than that of a silicon oxide film.
 7. Thenonvolatile semiconductor memory device according to claim 3, wherein athickness of the degraded charge film is less than a distance from alower surface to an upper surface of the charge film.
 8. The nonvolatilesemiconductor memory device according to claim 1, wherein the elementisolation insulating film includes PSZ (Polysilazane) or NSG (Non DopedSilicate Glass).
 9. A nonvolatile semiconductor memory device,comprising: a semiconductor substrate; element isolation insulatingfilms being along a first direction in the semiconductor substrate witha certain spacing therebetween and reached to a certain depth from asurface of the semiconductor substrate; and element regions sandwichedby the element isolation insulating film, with floating gate type memorycells disposed on an upper surface thereof, each of the floating gatetype memory cells comprising: a tunnel insulating film disposed on oneof the element regions; a floating gate disposed on the tunnelinsulating film, having an upper surface higher than an upper surface ofthe element isolation insulating films; an inter-gate multilayer filmdisposed continuously on the element isolation insulating films and thefloating gate; and a control gate electrode disposed on the inter-gatemultilayer film, the inter-gate multilayer film comprising a firstinter-gate film and a second inter-gate film disposed on the firstinter-gate film, and the second inter-gate film comprising: a sidesurface portion disposed on a side surface of the floating gate andconfigured from a material having a dielectric constant higher than thatof the first inter-gate film; and an upper surface portion disposed onthe element isolation insulating films and having fewer charge trapsthan the side surface portion.
 10. The nonvolatile semiconductor memorydevice according to claim 9, wherein the upper surface portion is formedfrom a material oxidized the second inter-gate film.
 11. The nonvolatilesemiconductor memory device according to claim 9, wherein the firstinter-gate film is formed from a silicon oxide film and the secondinter-gate film at the side surface portion is formed from a siliconnitride film.
 12. A method of manufacturing a nonvolatile semiconductormemory device, comprising: forming a plurality of element isolationtrenches along a first direction with a certain spacing therebetween,the plurality of element isolation trenches reaching a certain depthfrom a surface of a semiconductor substrate; filling the elementisolation trench with an insulating film including an oxidizing agent,thereby forming an element isolation insulating film; forming a tunnelinsulating film of a MONOS type memory cell on a surface of an elementregion, the element region divided by the element isolation insulatingfilm; forming a charge storage film of the MONOS type memory cellcontinuously on the element region and the element isolation insulatingfilm; and using heat treatment to diffuse the oxidizing agent includedin the element isolation insulating film and oxidize the charge storagefilm formed on the element isolation insulating film, thereby forming adegraded charge film.
 13. The method of manufacturing a nonvolatilesemiconductor memory device according to claim 12, wherein the degradedcharge film is formed using heat treatment to diffuse the oxidizingagent included in the element isolation insulating film and oxidize thecharge storage film formed on the element isolation insulating film asfar as an upper surface of the charge storage film.
 14. The method ofmanufacturing a nonvolatile semiconductor memory device according toclaim 12, wherein the degraded charge film is formed using heattreatment to diffuse the oxidizing agent included in the elementisolation insulating film and oxidize the charge storage film formed onthe element isolation insulating film, while leaving an upper surface ofthe charge storage film unoxided.
 15. The method of manufacturing anonvolatile semiconductor memory device according to claim 12, wherein,prior to forming the degraded charge film, a block insulating film and acontrol gate electrode are sequentially stacked on the charge storagefilm.
 16. The method of manufacturing a nonvolatile semiconductor memorydevice according to claim 12, comprising: prior to forming the elementisolation insulating film of the semiconductor substrate, forming a gateinsulating film and a gate electrode of a peripheral transistor;performing ion implantation to the element region of the semiconductorsubstrate through the gate insulating film and the gate electrode of theperipheral transistor; and subsequent to forming the degraded chargefilm of the MONOS type memory cell, forming a block insulating film anda control gate electrode of the MONOS type memory cell sequentially onthe charge storage film.
 17. The method of manufacturing a nonvolatilesemiconductor memory device according to claim 12, wherein the tunnelinsulating film is formed by oxidizing the surface of the elementregion, the element region divided by the element isolation insulatingfilm.
 18. The method of manufacturing a nonvolatile semiconductor memorydevice according to claim 12, wherein the charge storage film is asilicon nitride film.
 19. The method of manufacturing a nonvolatilesemiconductor memory device according to claim 12, wherein the chargefilm includes a high dielectric constant film having a dielectricconstant higher than that of a silicon oxide film.
 20. The method ofmanufacturing a nonvolatile semiconductor memory device according toclaim 12, wherein the element isolation insulating film includes PSZ(Polysilazane) or NSG (Non Doped Silicate Glass).